1. Field of the Invention
The present invention relates to an objective lens driving device and an optical disc apparatus using the objective lens driving device.
2. Description of Related Art
One type of objective lens driving device for use in an optical disc apparatus is disclosed in Japanese Laid-open Utility Model Application No. Hei-2-35330.
FIG. 9 shows the constitution of the objective lens driving device, FIG. 10 is a cross-sectional view of the objective lens driving device which is taken along a line G--G of FIG. 9, and FIG. 11 is a view of FIG. 9 which is taken along the direction of an arrow H.
An objective lens 7 is provided in a lens holder 3, which is supported and guided by a sliding shaft 6 which is slidably and rotatably disposed substantially in parallel to the optical axis direction of the objective lens 7. A focusing coil 5 is wound around the outer peripheral surface of the lens holder 3, and tracking coils 4 are provided at predetermined intervals in the peripheral direction on the outer peripheral portion of the lens holder 3. A magnetic circuit comprising a magnet 2, an outer yoke 1 and an inner yoke 9 is provided to generate magnetic flux so that the magnetic flux traverses the tracking coils 4 and the focusing coil 5. The tracking coils 4 and the focusing coil 5 are supplied with current from current supply means such as a flexible printed circuit (hereinafter referred to as "FPC").
During a focus control operation, the electromagnetic force acts in the direction of the sliding shaft 6 by causing the current to flow into the focusing coil 5 in the forward or reverse direction to move the objective lens 7 in the optical axis direction in accordance with the plane vibration of the recording surface of the optical disc, whereby the spot of the optical beam 8 can follow the recording surface of the optical disc. Further, during a tracking control operation, the electromagnetic force acts as a rotational couple of forces on the outer periphery of the sliding shaft 6 by causing the current to flow into each of the tracking coils 4 in the forward or reverse direction, whereby the spot of the optical beam 8 can follow the eccentricity of the track of the optical disc.
Next, the construction of a magnetic spring in a conventional objective driving device will be described by exemplifying an objective lens driving device disclosed in the official gazette for KOKOKU No. Hei-7-31814.
FIGS. 12 and 13 show an objective lens driving device having one objective lens which is used in a conventional CD apparatus or CD-ROM apparatus, wherein FIG. 12 is a diagram showing the constitution of the objective lens driving device and FIG. 13 is a cross-sectional view of the objective lens driving device which is taken along a line K--K of FIG. 12.
In FIGS. 12 and 13, reference numeral 21 represents an objective lens for focusing an optical beam, reference numeral 22 represents a lens holder, reference numeral 28-1, 28-2 represents a focusing coil, reference numeral 29-1, 29-2 represents a tracking coil, reference numeral 25 represents a sliding shaft, reference numeral 23 represents an inner yoke, reference numeral 24 represents an outer yoke, reference numeral 26-1, 26-2 represents a focusing magnet, reference numeral 27-1, 27-2 represents a tracking magnet, and reference numeral 30-1, 30-2 represents a magnetic substance for positioning the objective lens.
The focusing magnet 26-1, 26-2 is magnetized to have bipolarity in the height direction, and the tracking magnet 27-1, 27-2 is magnetized to have bipolarity in the peripheral direction. The magnetic density distribution in the peripheral direction in the neighborhood of the magnetic substance 30-1, 30-2 which confronts the focusing magnet 26-1, 26-2 is maximized at the center of the magnet, and thus the magnetic substance 30-1, 30-2 is magnetically balanced and stable at the position confronting to the center of the focusing magnet. Paying attention to the flow of the magnetic flux in the height direction, the magnet substance 30-1, 30-2 is magnetically balanced and stable in the neighborhood of the boundary of the N and S poles of the focusing magnet 26-1, 26-2 so as to form a magnetic loop of the N pole of the focusing magnet 26-1, 26-2.fwdarw.the magnetic substance 30-1,30-2.fwdarw.S pole.fwdarw.N pole. Accordingly, the objective lens 21 can be stably positioned by the magnetic balance in the rotational direction (tracking direction) and the height direction (focusing direction) with respect to the sliding shaft 25.